November 23, 2012

Eocene Bird Was A Giant But Peaceful Herbivore

When scientists first discovered fossils of the Eocene bird Diatryma in the mid 19th century, they portrayed the 7-foot avian as a fierce predator, which caught on with science writers and popular culture.

However, a recent discovery has suggested that this flightless giant was a gentle herbivore and not a flesh-eating terror as previously suggested.

According to a report in the journal Paleontology, a set of 50 million-year-old footprints made by the giant bird show that it had short toenails, not the razor-sharp talons possessed by birds of prey.

"This argues against an animal that catches prey and uses claws to hold it down,” study co-author David Tucker, from Western Washington University, told BBC Nature. “Carnivorous birds all have sharp, long talons.”

When palaeontologists began studying Diatryma fossils over 150 years ago, they concluded that the giant bird was a predator because of its size and large beak. The fact that the first Diatryma skeleton found in the United States was buried next to the bones of tiny horses and other potential prey only fueled this perception.

The reputation persisted over the years and paleontologists Larry Witmer and Kenneth Rose backed-up the bird´s bloodthirsty reputation in their 1991 research paper. They said that the skull of Diatryma was overbuilt for feeding on leaves and might have evolved as a need to break apart bones.

Those arguing against a carnivorous nature cited its relatively short legs, suggesting it could not have run fast enough to capture prey. They also noted that the bird did not have a hook on the end of its beak, unlike all raptors, which use it to hold their prey and tear into a carcass.

A 2009 landslide in northwestern Washington´s Chuckanut Formation led to the discovery of the study´s footprints that were linked to Diatryma. The geological activity also revealed several mammal and bird tracks that had been preserved in a 53 million year old Eocene-era riverbed.

Paleontologists found 18 large, three-toed imprints in Mesozoic age rocks. The scientists wrote in their report that “tracks of this size and shape would likely be interpreted as having been made by a small dinosaur.”

However, because they were found in Eocene strata, they were linked to Diatryma. The scientists couldn´t say with absolute certainty that the tracks were made by the big bird, but said it was the most likely candidate.

An analysis of the fossilized footprints showed they matched the anatomy of Diatryma, despite some previous artists´ renditions to the contrary. The analysis also showed broad feet supported by a heel pad. This suggested they were made by a walker rather than a swift runner, supporting the herbivore hypothesis, or at least conceding that it consumed meat as a scavenger.

The team said they believe the similarities between Diatryma and those of the meat-eating South American Phorusracids or "terror birds" led many paleontologists to assume that the two were somehow connected.

"The common belief that Diatryma,” the study said, “was likewise a carnivore is more a result of guilt by association than actual anatomical evidence.”